New clothes, a new haircut or a new perfume. Maybe you should have a look at his cell phone bill... if there is a number quite often, try to find out who belongs to the number.
your girlfriend
I doubt she would be your "girlfriend" if that was the case.
your girlfriend will turn away or will never go in for a kiss
please be my girlfriend again
Stare at her. Tell her. Be smart
He's trying to get you to get his girlfriend to break up with him
your communication partern or her behaviour towards you.
It sounds like there are no signs.
she calls, mails you and ask you out again.
Ask him or throw out subtle signs and see how he reacts.
Instead of looking for signs that your crush has a girlfriend or not why dont you just ask him. He can only say yes or no. Either way its a win win situation. If he does then it lets you move on and think well he was not worth it. If he does not then it gives you the opportunity so strike up a conversation and ask him out on a date. I hope this helps you.
I'd say see how he is acting around a certain girl. I'm sorry but That's all I can help you with.